This invention relates to a numerical control method and apparatus and, more particularly, to a numerical control method and apparatus in which flawing and attendant loss of a workpiece can be prevented by establishing and then prestoring in memory the areas which a movable member is forbidden to enter, and by adopting an arrangement wherein movement of the movable member is halted instantaneously as soon as the movable member enters the forbidden area during a machining operation.
Numerically controlled machine tools possess a movable member such as a tool or a table which is transported by means of a ball screw, and such factors as the length of the ball screw determine the maximum area over which the movable member can be transported. Since the movable member may inadvertently be caused to exceed the boundary of said area (a phenomenon referred to as "over-travel") by a programming error or malfunction, it is conventional practice to detect the overtravel as soon as it occurs and then bring the movable member to an emergency stop. To achieve such overtravel detection, it is customary to provide limit switches at the points of maximum stroke on the +X, -X and +Z, -Z sides of the area of allowable movement, and to provide a dog on the movable member, the arrangement being such that the dog presses against a limit switch when the movable member overtravels. The actuated limit switch produces a signal indicative of the overtravel event.
It is quite common in systems of the type described above to have a smaller area, within the area of allowable movement mentioned above, which the movable member is forbidden to enter. One example of such is the area defined within the final shape of the workpiece, that is, the final shape into which the workpiece is to be machined, referred to as the "commanded shape". For a tool to penetrate this area to a very small degree would cause only a negligibly small drop off in cutting precision, but penetration beyond a predetermined amount would cause the tool to cut too deeply into the workpiece and render it useless as a final product. Wasting workpieces in this manner obviously represents poor economy since the materials are costly.
Another example of a forbidden area is that occupied by the chuck provided on a lathe. A tool caused to penetrate this area would strike the chuck and result in damage or breakage of both the tool and chuck.
Such inadvertent movement of a movable member as described above can be caused by a malfunction in the arithmetic unit of a numerical control device, by an error in an NC tape read operation, by operator error, or by failure in a servo system. The prior art has been conspicuously devoid of truly satisfactory means for detecting penetration of a tool into a forbidden area. For example, one expedient has been to attach a sensor or limit switch to the chuck in order to detect contact between it and the tool. However, attaching the sensor or limit switch to the chuck is a difficult operation, the chuck takes on a more complicated structure, and costs are raised. Still other defects encountered with this expedient are that it is difficult to change or redefine the forbidden areas, and that it is not possible to detect tool penetration of the area defined by the commanded shape, even if the sensors or the like are mounted on the chuck.